1. Field of the Invention
The present invention relates to an FET switch used for switching between a plurality of transmission paths each having at least one FET (Field Effect Transistor).
2. Description of Related Art
In an antenna switch used in a digital cordless telephone or in a digital mobile telephone, both transmitting and receiving communication is carried out by means of a single antenna. An FET switch is used to select either one transmission path dedicated to the transmission of a relatively weak signal received by the antenna or another transmission path dedicated to the transmission of medium power (about 100 mW) to the antenna.
FIG. 1 is a drawing illustrative of a configuration of an FET switch of the prior art, wherein a first transmission path 61 has an FET 62 and an FET 63, and a second transmission path 71 has an FET 72 and an FET 73. The FET 62, FET 63, FET 72 and FET 73 are manufactured under the same process conditions. The FET 62 and FET 72 are identical FETs, and the FET 63 and FET 73 are identical FETs. Hence the first transmission path 61 and the second transmission path 71 have FET circuits of configurations symmetrical with each other.
FIG. 2 is a drawing illustrative of another configuration of an FET switch of the prior art, wherein a first transmission path 81 has an FET 82; an FET 83 and an FET 84, and a second transmission path 91 has an FET 92, an FET 93 and an FET 94. The FET 82, FET 83, FET 84, FET 92, FET 93 and FET 94 are manufactured under the same process conditions. The FET 82 and FET 92 are identical FETs, the FET 83 and FET 93 are identical FETs and the FET 84 and FET 94 are identical FETs. Hence the first transmission path 81 and the second transmission path 91 have FET circuits of configurations symmetrical with each other.
As described above, FET switches of the prior art have transmission paths made in symmetrical FET circuits and Further the FETs constituting each transmission path are manufactured under identical process conditions, For the purpose of universality.
In case a switching circuit is mated with another device such as amplifier, mixer, etc. operating in the microwave region, a switching circuit which matches the characteristics of the integrated circuits of the mating device is selected from switching circuits manufactured separately from the mating devices, and the selected switching circuit is combined with the mating device thereafter.
However, as the degree of circuit integration increases, the switching circuit is increasingly integrated with other devices such as amplifier and mixer into MMIC (Microwave Monolithic Integrated Circuit). This leads to such problems that the FET circuit has optimum performance for the characteristics of one transmission path but is not suitable for the characteristics of the other transmission path, or the FET circuit has somewhat acceptable performance for both transmission paths but are not satisfactory, if the FET circuit has a symmetrical configuration in both transmission paths and further identical FETs are used in both transmission paths, as in the case of the FET switch of the prior art.